When subjects selectively attend to visual stimuli, the event-related potentials (ERPs) elicited by attended stimuli exhibit early sensory-evoked Pl (100 ms) and Nl (180 ms) peaks of enhanced amplitude. This pattern suggests that the mechanism of spatial selective attention involves, in part, an increase in information flow in the visual pathways via a 'gating' or 'gain control, process. At present, however, only minimal information is available regarding either the neural generators of these early attention-sensitive ERP peaks or the relationship between ERP attention effects and perceptual processing. Proposed are a series of ERP experiments investigating the mechanisms of visual selective attention. First, in healthy subjects, the neural bases of spatial selection will be investigated by analysis of (i) scalp voltage and current density distributions and (ii) dipole models of the attention-sensitive Pl and Nl peaks elicited by different classes of visual stimuli. Second, the perceptual significance of the early, attention-related ERP enhancements will be investigated in a series of experiments that seek to correlate signal detection measures (d') with the magnitudes of ERP attention effects. Third, experiments will investigate the spatial selectivity of the early Pl and Nl attention effects. ERPs to letter stimuli will be compared as functions of their (i) distance from the current spatial focus of attention in two-dimensional space and their (ii) similarity to the attended-location events in color. Lastly, related experiments in commissurotomy patients will investigate (i) the contributions of callosally mediated activity to the attention-sensitive ERPs and (ii) the hemispheric specialization of color selection. Understanding the basic mechanisms of selective attention will aid in identifying the physiological bases of cognitive and mental disorders such as schizophrenia, attention deficit disorder, dementia and learning disability.